New York City Mayor Bill de Blasio said public schools will be closed Tuesday. Massachusetts Gov. Charlie Baker urged people to avoid driving. About 4,000 flights that were scheduled for Tuesday at the area’s biggest airports have been canceled.

“Everyone along the east coast be safe and listen to local officials as a major winter storm approaches,” President Donald Trump tweeted.

Just one week before the official start of spring, the National Weather Service issued blizzard warnings and watches for the region, including northeastern New Jersey, southeastern New York and southern Connecticut, as well as Boston, Philadelphia, Baltimore and Washington, DC.

New York City could get up to 20 inches of snow, De Blasio said, as well as coastal flooding and wind gusts as high as 40 to 50 mph. Parts of Massachusetts could see 24 inches or more and similar powerful winds, Baker said. The region is also expecting downed power lines and service interruptions.

“This should be a very serious blizzard, one that everyone should take seriously,” de Blasio said.

The snow is expected to begin after midnight in New York City. The period of greatest accumulation should be between 6 a.m. and noon Tuesday, with a possible 2 to 4 inches per hour and whiteout potential during that time, de Blasio said.

He said it’s too soon to say how subways and buses will be affected.

Baker provided similar details about what’s expected in Massachusetts, saying the state expects sleet, rain and extremely cold temperatures along the coast.

“This is going to be a lot of snow and it’s going to be a mess,” Baker said.

New York Gov. Andrew Cuomo activated the State Emergency Operations Center for Monday evening. State agencies have already deployed personnel, assets, and state stockpile resources — including sandbags, generators and pumps — to key areas.

Cuomo urged commuters to drive carefully and avoid unnecessary travel. Motorists, especially tractor trailers, should be prepared for road closures across the state, he said.

Travelers should expect delays. By 2 p.m. Monday, airlines had canceled 2,061 flights that had been scheduled to depart Tuesday from eight major airports in the storm’s path, as well as 1,810 flights that were scheduled to arrive that day, according to FlightAware. (Keep up with the latest cancellations here.)

American Airlines canceled more than 450 flights Monday and 1,450 Tuesday due to the storm. The alert covers 40 airports — including hubs in Chicago, New York, Philadelphia and Washington, DC. Passengers may rebook without change fees, the airline said.

Amtrak said it will operate a modified schedule in the Northeast region on Tuesday. It advised passengers with reservations to monitor conditions and make changes before their scheduled departure using Amtrak.com or the company’s mobile apps.

It’s a winter wonderland on social media

From the Midwest to New England, Americans used social media to share images of the severe weather.

An Instagram post captured a crewman at Chicago’s Midway International Airport de-icing an early flight.

Also in the Windy City, the Chicago Loop sculpture made a striking image in the snow.

Shoppers began tweeting photos of long lines and empty shelves at grocery stores.

On Twitter, the storm (dubbed “Stella” by the Weather Channel) generated memes of Marlon Brando despairingly calling for “Stella!” in the classic movie “A Streetcar Named Desire.”

No ‘year without winter’ after all

Ahead of the storm, bone-chilling temperatures persisted across the region.

“Nearly one in every three people in the US are under a winter weather alert of some sort,” CNN meteorologist Pedram Javaheri said. “This all comes after what seemed like it would be the year without a winter.”

Less than two weeks ago, the nation’s capital was enjoying 80-degree temperatures.

Forecasters expect Washington will get off easy, with 5-10 inches of snow.

“The first low begins in the Midwest and progresses east to the Mid-Atlantic region. The second low begins off the coast of Florida and moves north along the east coast and meets up with the first low around Washington D.C.”

“Since January first, we have seen over 9,000 record high temperatures set in the US — compare that to only 1,300 record low temperatures this winter, a 9 to 1 ratio favoring warmth,” Javaheri said.

Despite the advance of colder temperatures across the Eastern US, last month was one of the warmest on record, the National Weather Service tweeted.